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đźš« Stop Writing Fat Controllers: Follow the Controller-Service-Repository Pattern
In a typical Spring Boot project, it’s easy to let your controller classes grow out of control. You start by handling a simple request, then add a bit of business logic, maybe some database access — and before you know it, your controller has become a bloated, untestable mess.
If your controller methods are more than 10–15 lines, or if they contain database logic, conditionals, and exception handling — you’re likely writing a fat controller.
The solution? Follow the Controller-Service-Repository (CSR) pattern — a clean and proven way to structure Spring Boot applications that keeps your code readable, testable, and maintainable.
💣 What’s a Fat Controller?
A “fat controller” refers to a controller class that does too much:
- Handles request mapping
- Contains business logic
- Talks to the database
- Manages error handling
- Performs input validation
- Builds complex response objects
Let’s look at an example:
@RestController
@RequestMapping("/products")
public class ProductController {
@Autowired
private ProductRepository productRepository;
@PostMapping
public ResponseEntity<String> createProduct(@RequestBody Product product) {
if (product.getPrice() < 0) {
return ResponseEntity.badRequest().body("Invalid price");
}
product.setCreatedAt(LocalDateTime.now());
productRepository.save(product);
return ResponseEntity.ok("Product created");
}
}This might look fine — but it mixes:
- Input validation
- Business logic
- Persistence logic
- Response formatting
It’s hard to test, hard to extend, and violates the Single Responsibility Principle.
